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<p>Photoacclimation strategies and sensitivity to photoinhibition were determined in natural phytoplankton assemblages during a phytoplankton bloom in the Amundsen Sea (Southern Ocean) in relation to community composition, pigment content, light, and iron (Fe). Non-photochemical quenching (qN) was measured during recovery after surface irradiance exposure (SIE) for 20 min. The qN was separated into slow (qI, photoinhibition through damage of Photosystem II) and fast (qE, xanthophyll cycling) relaxing components. Phytoplankton within the upper mixed layer (UML) showed a higher ratio of photoprotective xanthophyll cycle pigments (diadinoxanthin [DD] + diatoxanthin [DT]) to chlorophyll a (chl a), indicative of acclimation to high light, which resulted in lower photoinhibition after SIE when compared to phytoplankton residing below the UML. Within the UML, we found differences in photoacclimation strategies in assemblages dominated by Antarctic diatoms versus Phaeocystis antarctica (Haptophyta). Diatoms had a higher ratio of (DD + DT)/chl a, and the ratio tracked mean light levels within the UML, whereas this relationship was not apparent in P. antarctica, which had a lower (DD + DT)/chl a ratio. Despite these differences, diatoms and P. antarctica exhibited similar degrees of qN that were dominated by qE with very little qI. Bioassays under high and low Fe concentrations revealed an increase in the (DD + DT)/chl a ratio in Fe-limited populations dominated by diatoms and decreased photoinhibition. In experiments dominated by P. antarctica or with mixed populations, acclimation to low Fe increased the (DD + DT)/chl a ratio in most experiments; however, this did not affect photoinhibition. This study shows that under in situ conditions in the Amundsen Sea (1) phytoplankton photo acclimation efficiently minimizes photoinhibition, (2) photoinhibition does not control the relative abundances of P. antarctica or Antarctic diatoms, and (3) Fe limitation does not increase photo inhibition of either P. antarctica or Antarctic diatoms.</p>

Photoacclimation and non-photochemical quenching under in situ irradiance in natural phytoplankton assemblages from the Amundsen Sea, Antarctica